For many device applications, an input signal must be pre-processed within the device so that it is optimized for device-specific technology that performs the primary functions; similarly, prior to transmission to the outside world, the signal emerging from the device core must be post-processed to produce a signal that is compatible with typical user requirements. For optoelectronic modules, the required optical signal processing includes functions such as light generation, wavelength control, polarization control, phase control, beam steering, beam shaping, beam splitting or recombination, modulation, and detection. For ease of use, or for control of parameters that are critical to device performance, it is often desirable to integrate many pre- and post-processing functions inside the module itself For example, a significant benefit is that optical insertion loss of the device can often be reduced by integrating more optical functions into the module. This occurs because component selection can be more easily optimized for the device-specific technology, and also because the number of physical connections between different devices or modules has been reduced. A low-loss optoelectronic module is enabling for system applications, as it can be more easily deployed at different points in a system and lengthens the span associated with the system. In addition, the physical dimensions or “footprint” of the device can be shrunk by component integration.
Integration of the pre-processing and post-processing optical functions is especially critical for silicon-based optoelectronic circuits operating at infrared wavelengths. Since silicon-based lasers are not yet widely available for commercial applications, it is not currently possible to incorporate the light source in the same silicon wafer as the signal processing and receiver elements. Thus, the light signal must be introduced to the silicon waveguide from an external source. This requires intervening optical elements (between the source and the waveguide) to pre-process the signal so that a substantial fraction of the light intensity can be transferred to the waveguide. In addition, because silicon-based detectors appropriate for infrared wavelengths are just beginning to be developed, the light signal must be transferred from the silicon waveguide to an external detector or receiver element. Thus, optical elements are also required on the output side of the device for post-processing of the optical signal.
Exemplary methods of coupling light into high-index contrast waveguides known from prior art include prism couplers, grating couplers, tapered mode converters, and specially shaped fiber terminations or lensed fibers. While all these optical elements have been used in laboratory environments to transfer a fraction of the light available from an external source into a high-index contrast waveguide, significant limitations arise when these components are incorporated into low-loss device prototypes or finished product.
For example, the smallest spot size that can be produced by specially shaped fiber terminations, lensed fibers, or tapered mode converters is on the order of 1.5 μm, which is incompatible with the sub-micron dimensions of some silicon waveguides. In particular, a number of applications require single-mode silicon waveguides that have dimensions on the order of 0.35 μm or less. The mismatch in the mode-field diameters of the output beam of the specially prepared fiber or tapered mode converter and that of the waveguide mode results in very high insertion loss. Even if the waveguide thickness is on the order of a few microns, the requirement that input and output ports for devices must be located at edge facets of the wafer die containing the waveguide imposes significant restrictions on device geometry (for example, device topology and size) when coupling to specially prepared fibers or tapered mode converters.
The restrictions discussed above can be relaxed by employing either grating couplers or prism couplers to couple light from an external source into and out of high-index contrast waveguides. With appropriate designs, it is possible to successfully couple light into waveguides with thicknesses ranging from tens of nanometers to tens of microns. Moreover, the grating or prism element can be placed at any desired location on the die or wafer surface, facilitating optical access to a considerable fraction of the die or wafer.
Despite these considerable advantages, fabrication difficulties associated with both grating and prism couplers have limited their use to a few specialized applications. The coupling efficiency of grating couplers is relatively sensitive to the grating period, depth, and slant or tilt angles. Theoretically, coupling efficiencies on the order of 70-80% can be achieved if the design targets for the grating parameters are met; in practice, coupling efficiencies on the order of 40% are more commonly observed due to sensitivity to fabrication tolerances.
In the prior art, prism couplers have required that a large bulk-optic element (several mm in dimension) be placed in very close proximity to a waveguide and oriented very precisely with respect to the waveguide. In this context, “very close proximity” is intended to mean that the separation distance between the optical element and the waveguide permits evanescent coupling of light from the optical element to the waveguide. For infrared wavelengths used in telecommunications applications, a typical value of the separation distance falls in the range of 200-550 nm. The required motion control to manipulate the prism relative to the waveguide (for example, using piezoelectric mounts) can be accomplished in a laboratory optical bench or test set-up, but such methods cannot be implemented in a compact optoelectronic package. For this reason, prism coupling applications have been primarily limited to waveguide test and characterization.
Since prism couplers have not been utilized in compact optoelectronic packages in the prior art, optical and mechanical assemblies that are appropriate for use with prism couplers housed in small device structures have not been developed. For example, specific embodiments of exemplary optical elements that deliver light to and receive light from a prism coupler device in a compact optoelectronic package have not been disclosed in the prior art. In a laboratory setting, the optical elements can generally be manipulated or “tuned” in a number of ways to optimize signal transmission when the signal that is introduced to the prism coupler is changed in some way (such as in wavelength, polarization state, beam position, angle of incidence, etc.). For a packaged device, it is preferable to design a device that is transparent to a variety of inputs; that is, limited tuning (or perhaps no tuning) of a small subset of parameters, is required in order for the device to function properly when the input state of the signal is varied. Thus, the choice of optical parameters associated with the input and output beams, the input and output optical elements, and the prism coupler directly impacts the versatility and manufacturability of the device. However, because prism couplers have not been packaged in compact optoelectronic devices in the prior art, specific designs that produce versatile and manufacturable devices have not yet been developed.
Thus, a need remains in the art for designs and implementations of optical systems that can be interfaced to prism couplers in compact, low-loss, and robust optoelectronic packages.